1. Field of the Invention
The present invention relates to a vertical type heat-treatment apparatus.
2. Description of the Related Art
In a manufacturing process of semiconductor devices, a furnace is used to form an insulating film, a compound semiconductor thin film, an organic metal thin film, and the like on a semiconductor wafer as an object to be treated. Such a furnace is normally designed to treat 100 products or more at once so as to improve productivity. Furnaces can be classified into horizontal type furnaces which are horizontally installed and vertical type furnaces which are vertically installed. A great deal of attention has been paid to vertical type furnaces because installation space can be reduced.
A vertical furnace, for example, has the following arrangement. The vertical furnace has a loading mechanism for vertically moving a boat. A large number of wafers are mounted on the boat at predetermined intervals. A reaction furnace is arranged above the loading mechanism.
The vertical type furnace is normally arranged in a clean room. The clean room supplies clean air to the vertical type furnace so as to prevent impurities from adhering to wafers. An enormous cost is required to keep high cleanliness of a clean room. For this reason, a limited space in the clean room must be effectively used. From this point of view, the front surface portion of the vertical furnace is placed to oppose the inside of the clean room, and other portions are placed in a room which is adjacent to the clean room and has a low cleanliness.
Clean air, however, normally flows downward in the clean room to form a downflow. For this reason, the vertical type furnace becomes an obstacle to the flow of air, and it is difficult for clean air to reach the loading mechanism located below the furnace. Hence, a fan mechanism for clean air is independently arranged in the installation area (to be referred to as a loading area hereinafter) of the loading mechanism. By using this fan mechanism, clean air is blown into the clean room through the loading area. As a result, impurities such as dust in the loading area are blown into the clean room. Since the flow of clean air in the clean room is a downflow, the air containing the impurities blown from the fan mechanism and passing through the loading area collides with the floor of the clean room. As a result, the impurities blown from the loading area into the clean room are discharged toward the floor of the clean room.
The vertical type heat-treatment apparatus comprising the vertical type furnace having the above described arrangement, however, has the following problems:
(1) If a special clean air fan mechanism is arranged to remove impurities in the loading area, the overall apparatus becomes complicated and is increased in size.
(2) Turbulence may occur near the vertical type furnace depending on a flow rate of clean air sent from the special fan mechanism, and impurities reach components such as the vertical type furnace in the clean room, resulting in adverse effects.
In order to solve these problems, the following countermeasure has been taken.
A clean room is normally set to have an atmosphere having a positive pressure higher than that of a maintenance room which is adjacent to the clean room and is used to perform maintenance and the like. A flow path is formed in the clean room by utilizing the difference in pressure between these atmospheres so as to cause clean air flowing downward to pass the loading area and introduce it to the maintenance room.
In this case, however, air from the maintenance room may flow into the loading area to cause turbulence or the like depending on a difference in pressure between the clean and maintenance rooms or the size of a clean air flow path. For this reason, impurities which are present in the loading area cannot be reliably removed, and the vertical furnace and the like may be adversely affected.